The JPEG baseline method for still image coding uses the Discrete Cosine Transform (DCT) in a fixed 8×8 pixel partition. Through a linear quantization table and zig-zag scanning of DCT coefficients, the redundancy and band width characteristics of the DCT are exploited over a range of compressions. Recently, however, it has become clear that the JPEG coder is not particularly efficient at higher compression ratios, and other methods such as wavelets have produced better results while having the advantage of being fully embedded. Some researchers have also attempted to combine DCT with zerotree quantization, usually associated with wavelet transforms: see Xiong, Guleryuz and Orchard, ‘A DCT-Based Image Coder’, IEEE Sig. Proc. Lett., Vol 3, No 11, November 1996, p289.
It is an object of the present invention to advance the field of image compression generally, and in particular to provide an improved method of image compression which is capable of use with well understood transforms such as the DCT.